Glow discharge decomposition is employed for the preparation of thin films of a variety of materials such as semiconductor materials, insulating materials, optical coatings, polymers and the like. In a typical glow discharge deposition, a process gas which includes a precursor of the material being deposited, is introduced into a deposition chamber, typically at subatmospheric pressure. Electromagnetic energy, either AC or DC, is introduced into the chamber and energizes the process gas so as to create an excited plasma therefrom. The plasma decomposes the precursor material and deposits a coating on a substrate maintained proximate the plasma region. Frequently the substrate is heated to facilitate growth of the deposit thereupon. This technology is well known in the art. Early glow discharge depositions employed either direct current, low frequency alternating current or radio frequency alternating current to energize the plasma; radio frequency current is still very widely employed for this purpose.
One particular drawback to certain prior art glow discharge deposition processes was their relatively low speed, and in an attempt to increase deposition rates those of skill in the art turned to the use of microwave energized plasmas. It was found that microwave energized glow discharge processes provided very high deposition rates. Initially, process parameters derived from radio frequency energized depositions were applied to microwave depositions; however, it has been found that plasma conditions encountered in a microwave process differ from those in radio frequency energized processes and hence necessitate changes in the various process parameters.
One important class of semiconductor materials which are manufactured by plasma deposition processes are the Group IV semiconductor alloys. Most typically these materials comprise alloys of silicon and/or germanium together with alloying, modifying and dopant elements, the most typical of which are hydrogen, halogens and the Group III and Group V elements. It has been found that Group IV semiconductor alloys deposited in high rate microwave processes tend to incorporate more hydrogen than do comparable materials prepared under radio frequency plasma conditions. Hydrogen content is a particularly important parameter for these semiconductor alloys since hydrogen tends to increase the band gap of the materials thereby changing their optical and electrical properties. If these hydrogen rich materials are incorporated into photovoltaic devices it has been found that increased hydrogen content will decrease the short circuit current of the cell (J.sub.sc) and will increase the open circuit voltage (V.sub.oc) of the device. Generally, it has been found that photovoltaic devices which include group IV semiconductor layers prepared in accord with prior art microwave energized deposition processes have efficiencies which are lower than the efficiencies of similar devices which include radio frequency deposited group IV semiconductor alloys.
As noted above, the substrate in a glow discharge deposition process is typically heated to facilitate growth of the deposit and it has been found that substrate temperature is a parameter which has a direct influence upon quality of the deposited semiconductor material and hence the efficiency of photovoltaic devices manufactured therefrom. In radio frequency energized processes it has been found that substrate temperatures in the range of 200.degree. to 350.degree. C. and preferably 275.degree. to 300.degree. C. are preferred for the preparation of silicon and/or germanium thin film alloy materials.
When the art turned to the use of microwave energized depositions, process parameters, including substrate temperature, were adapted from radio frequency processes. For example, U.S. Pat. Nos. 4,504,518; 4,517,223 and 4,701,343 all describe microwave energized glow discharge deposition processes. These patents acknowledge that radio frequency processes typically employ substrate temperatures in the range of 227.degree.-327.degree. C. and broadly recite that the microwave processes taught therein can operate with substrate temperatures in the range of 20.degree. C. to 400.degree. C. and that a preferred substrate temperature range for the processes is 250.degree.-325.degree. C. It is specifically recited therein that the preferred temperature range for the deposition of intrinsic silicon and silicon-germanium alloys is 275.degree. C. and the preferred temperature range for the deposition of doped silicon alloy layers is 250.degree.-300.degree. C.
U.S. Pat. No. 4,715,927 teaches that silicon alloy materials may be prepared by a microwave process employing substrate temperatures of about 300.degree. C. and that silicon-germanium alloys are preferably prepared at substrate temperatures of 275.degree. C.
U.S. Pat. No. 4,470,369 teaches that p-doped, microcrystalline silicon alloy materials are prepared by a microwave deposition process employing substrate temperatures ranging from ambient to 275.degree. C. with the preferred range being 150.degree.-225.degree. C.
U.S. Pat. No. 4,729,341 teaches the microwave deposition of silicon alloy materials at a preferred substrate temperature of about 300.degree. C.
U.S. Pat. No. 4,515,107 shows the preparation of silicon alloy materials by a microwave process employing substrate temperatures of 350.degree. C.
U.S. Pat. No. 4,713,309 shows the manufacture of amorphous silicon xerographic drums in a microwave deposition process employing substrate temperatures which are preferably 225.degree. C.
It will be seen that prior art microwave deposition processes tend to follow radio frequency substrate temperature parameters and as such operate in a substrate temperature range in the neighborhood of 300.degree. C. and even in the broadest teachings never exceed 400.degree. C. The present invention resides in, and recognizes, the fact that plasma conditions in a microwave deposition process are uniquely different from those encountered in a radio frequency energized process. Substrate temperature ranges as taught in the prior art are inappropriate for microwave energized deposition processes. Improved semiconductor materials are obtained in a microwave energized deposition when the substrate temperature range is increased over that of the prior art. These and other advantages of the present invention will be readily apparent from the drawings, discussion, description and examples which follow.